1985 K100 RT.  Early model.

I have both the 7L (yellow) and 4L (red) warning lights on my speedo cluster, and all has been fine until today.

Now the 4L warning light comes on intermittently (and there's more than 4L in the tank, the yellow 7L is not even on yet).

I've read through posts here trying to find the solution, and can't find it.  There are references to this problem on early K100s.  Where should I start looking?  What would cause that darned 4L red warning light to come on when there's plenty of fuel?

Thanks!

The system uses a thermistor for each level and when the fuel is in contact with the thermistor it is cool and doesn't conduct and when the fuel drops to the level it is exposed so warms up and conducts bringing the light on.
Being that the light comes on when there is fuel to cool it I would say the thermistor could well be cactus.
You would need to get someone who is conversant with that side of electronics to test it.
You may find a good used sender but I would not be too hopeful as I dont think I have ever seen one for sale.
It might be easiest to just disconnect the light and use the tried and proven open the cap and check like I do regard less of what my light or gauge tells me.

I can't help you with the problem, but if you do need to replace the thermistor - good luck! I managed to buy a second-hand unit when mine fractured at the base and started leaking petrol - there are some on eBay, but rarely and of questionable quality. I think I got lucky with mine. The design of the early ones means you can't just replace one component - it's all or nothing.

Unless you're lucky, I think you'll be safer just going on manual checks and the trip meter.

Hi guys, This is a subject that has been dear to my heart the past couple of days. Mine not only doesn't works but it leaks!
I am new to posting pictures so please be kind.
the first picture is of the sender unit which is screwed into the underside of the tank
Having trouble with other pics so will try to get the important one even if is in another post

Part deux!!!!
  
OK, slightly better but I am trying.
There are 5 wires coming via the 4 prong the 2 long ones are the fuel pump power. note the stake on eyelets are different sizes. one is 8mm
the other is 7mm this is very thoughtful of BMW but I would have preferred not to have to figure this out myself.
the other 3 wires consist a short brown wire to the lowest thermistor and the return wires should correspond in length. these are soldered connections and in my case one was broken.
I did not think to measure any values in the thermistors which you can see as the small cylinder type objects that resemble resistors from the outside.
In my case the inner sleeve of the base had leaked between it and the outer sleeve both which are held on to the tank by the large nut.
Right now my sensor is curing a combination of JB weld topped by a fuel resistant gasket compound. I will test it tomorrow and report on my success :cyclops: 
This entire contraption is very flimsy and breakable (un-like the rest of the bike) so be careful of torque and tugging.
I hope this helps you as I could not find any direct pictures in my search.(probably the wrong places.
Cheers 
Rob

good work rob ...hope it comes good for you

cheers

and keeping in the same vein as this thread, how the heck do you keep the plug from falling out the bottom of this sensor? 

Seems like it plugs in, but the slightest touch would make it fall out.

orforester wrote:and keeping in the same vein as this thread, how the heck do you keep the plug from falling out the bottom of this sensor? 

Seems like it plugs in, but the slightest touch would make it fall out.

when i bought mine the previous owner had an on/off fuel pump problem.
i found that fuel pump/fuel level sensor plug had a very loose fit.
i gently squeezed up the 4 female connectors with a pick to snug them up.
been ok for the past 3k miles.

So you opened them up a little with a pick??  like made the Male plugs wider??

orforester wrote:So you opened them up a little with a pick??  like made the Male plugs wider??

nope,squashed the female one's smaller by slipping a pick between the plastic and metal connectors.
originally it was loose enough that just gravity and slight tap would make it come off !
what i did gave it enough drag/friction to hold on snug.
i read some place that there is a spare female connector just like it in the solenoid box under the tank.
i think it was for an optional alarm.

I had the same problem and used a small precision screwdriver to open out the male pins. It has never given trouble since.

kartooo wrote: i read some place that there is a spare female connector just like it in the solenoid box under the tank.
i think it was for an optional alarm.

Correct, the same connector is used for the OEM alarm.

I had the same issue with my cafe build.  replaced the plug on the cable end with one off an old C.B. radio microphone, it has the same configuration as well as a threaded knurled nut that screws over the tank gauge female fitting. It has worked now for a few years, never fallen off.

http://search.jaycar.com.au/search?w=4%20pin%20mic%20plug&view=list

http://www.ebay.com.au/itm/NEW-MICROPHONE-PLUGS-4-PIN-MIC-CB-RADIO-HAM-27mhz-MIKE-/190417511542?pt=AU_Electronics_Radio_Equipment&hash=item2c55c4a476



Good luck.

  smart one, bushy.

Inge K. wrote:  smart one, bushy.

+1
i am a ham and have a few of them kicking around.
got 1 next to me right now on my Ten-Tec Omin VI mic....